1. Field of the invention
The present invention relates to code assignment in multi-rate communications of the radio communication system employing Code Division Multiple Access (CDMA) system, where codes having tree structured orthogonal multi-spreading factor sequence are used as spreading codes.
2. Description of the Related Art
The system utilizing coherent multi code and the system utilizing tree structured multi-spreading factor sequence code have been introduced as a multi-rate transmission system in the radio communication system employing DS-CDMA (Direct Sequence-CDMA). In the multi-rate transmission utilizing the coherent multi code, plural codes are simultaneously used to perform multi-code-multiplexing. In the multi-rate transmission utilizing the tree structured multi-spreading factor sequence code, orthogonal codes having equivalent spreading factors are multiplexed in parallel. Receivers for receiving plural codes are needed for a mobile station used for the multi-rate transmission utilizing the coherent multi code. On the other hand, only one receiver is needed for the system utilizing the tree structured multi-spreading factor sequence code.
The conventional CDMA system using tree structured orthogonal multi-spreading factor sequences is disclosed in, for example, a technical report entitled xe2x80x9cMulti-rate Wideband DS-CDMA Radio Access For Next Generation Mobile Radio Systemsxe2x80x9d, the Institute of Electronics, Information and Communication Engineers, RCS-97-86, 1997. The conventional system will be explained with reference to FIG. 11.
FIG. 11 shows a configuration of the conventional tree structured orthogonal multi-spreading factor sequences.
The tree structure is composed of four levels: level 1 at the top through level 4 at the bottom, defined just for convenience. As there are four levels in this case, the channel transmission rate using the top level (level 1) code is eight times as fast as that of the channel using the lowest level (level 4) code. The channel transmission rate using level 2 code is four times the rate of channel using the level 4 code. The channel transmission rate using level 3 code is twice the rate of channel using the level 4 code. Cx(y) in FIG. 11 represents a kind of codes, x stands for a code level, y stands for an identification number in the same code level, and ◯ stands for a code.
In FIG. 11, a code in the level 1 can be used for communication of twice the transmission rate of level 2. Now, a concrete example will be explained referring to the case of tree structure as shown in FIG. 11. When C2(1) is assigned to a user of four times the transmission rate of the lowest rate (C4(x) level in FIG. 11), the codes C4(1), C4(2), C4(3), C4(4), C3(1), and C3(2) can not be assigned to other users because of keeping the code orthogonality. Due to this characteristic, all the lower level codes connected with an assigned code through branches and all the upper level codes connected with the assigned code through only upward branches are not-assignable. When C2(1) is the assigned code, all the lower codes C4(1), C4(2), C4(3), C4(4), C3(1) and C3(2) connected with the assigned code by branches are not-assignable, and the upper code C1(1) connected with the assigned code by the shortest branch is not-assignable, either.
As stated above, in a CDMA system using codes of tree structured orthogonal multi-spreading factor sequences, the code assignment for a mobile station is restricted because of the tree structure characteristic. Accordingly, in the case of FIG. 11, if C4(5) and C4(7) are assigned to calls of the lowest rate (level 4), it is impossible to assign a call having the rate of level 3 or higher than level 3. However, on the supposition that C4(6) is assigned to the call of level 4 instead of C4(7), it is possible to assign C3(4) to the call of level 3, and consequently, channels are effectively utilized.
In the multi-rate transmission of the conventional radio communication system employing Code Division Multiple Access (CDMA) system where codes having tree structured orthogonal multi-spreading factor sequence are used, when a code has been already assigned (in use), it is impossible to assign all the codes upper than the code. This causes a problem that the channel utilization is deteriorated. It is an object of the present invention to utilize all the system (all the branches) of the tree structure for assigning a code to be responsive to a new call, in order to enhance the channel utilization.
In the conventional art, a channel mismatch would occur at the call ending, even if a channel match was kept at the code assigning time of a new call starting. This sometimes causes a problem that selecting a code to be assigned to a new call is impossible. It is another object of the present invention is to avoid the channel mismatch by switching a code being used in communication. Consequently, call loss rate can be reduced.
In the code assignment of the conventional radio communication system employing CDMA system, it is necessary for a base station and a mobile station to simultaneously switch codes in order not to stop the communication under way. However, as the base station does not have means for judging a code switch timing over the mobile station, the base station is troublesomely needed to simultaneously perform reception and transmission using both the codes used before and after switching. It is another object of the present invention to reduce the stop time caused by code switching and the hardware needed for reception/transmission using both the codes between the base station and the mobile station, by means of sending the code switching timing from the base station.
In the code assignment of the conventional radio communication system employing CDMA system, there is a problem that much process is needed for synchronizing the code switch timings in the base station and the mobile station. It is another object of the present invention to reduce the code switching process by means of depending on mobile station""s own timing.
Namely, it is a general object of the present invention to provide an apparatus and a method for effectively utilizing the channel, comprising means in the base station to judge a code switch timing over the mobile station, and reducing the synchronization process.
According to one aspect of the present invention, a code assigning apparatus used in a radio communication system including a base station controlling apparatus, a plurality of base stations, and a plurality of mobile stations, employing CDMA (Code Division Multiple Access) method, which uses codes of tree structured orthogonal multi-spreading factor sequences as spreading codes, for radio access between the plurality of base stations and the plurality of mobile stations, and providing multi-rate transmission between the plurality of mobile stations and the plurality of base stations, the code assigning apparatus comprises a code selecting unit for selecting an assignable code which suits specifications requested by a new starting call and exists in a location as close to a not-assignable code as possible and in a level lower than the not-assignable code in the tree structure.
According to another aspect of the present invention, the code selecting unit of the code assigning apparatus comprises
a first selecting unit for selecting one or more than one codes corresponding to rate information included in a message from one of the plurality of mobile stations, as first codes,
a second selecting unit for selecting one or more than one first codes if they are assignable to the one of the plurality of mobile stations, as second codes,
a level detecting unit for retrieving codes upper than the second codes one by one, detecting not-assignable codes, which are assigned to another of the plurality of mobile stations, out of the codes upper than the second codes, as third codes, and detecting each level of the third codes,
a third selecting unit for selecting one of the second codes whose upper third code is in a lowest level in the tree structure, and
a code assigning unit for assigning the one of the second codes selected by the third selecting unit to the one of the plurality of mobile stations.
According to another aspect of the present invention, the code assigning apparatus comprises
a code-switching/code-assignment processing unit, when there is no assignable code which suits specifications requested by a new starting call, for making a code which is not-assignable and suits the specifications requested by the new starting call assignable by way of switching an assigned code being used to another code connected by a branch different from a branch used for the assigned code in the tree structure.
According to another aspect of the present invention, the code-switching/code-assignment processing unit of the code assigning apparatus comprises
a first selecting unit for selecting one or more than one codes corresponding to rate information included in a message from the second mobile station, as first codes,
a second selecting unit for selecting one or more than one first codes if they are assignable to the second mobile station, as second codes,
a fourth selecting unit, when there is no second code, for selecting one or more than one first codes if their lower codes have been assigned to the first mobile station, as fifth codes,
an assigned code detecting unit for detecting a code lower than the fifth codes and assigned to the first mobile station as a fourth code,
a switchable code detecting unit for detecting a code to which switchable from the fourth code, for the first mobile station, as a sixth code,
a code switch directing unit for directing the first mobile station to switch from the fourth code to the sixth code, and
an assigning unit for assigning a fifth code to the second mobile station after switching all the fourth codes to other codes.
According to another aspect of the present invention,
the code-switching/code-assignment processing unit includes a code switch informing unit for informing that the fourth code having been used by the first mobile station can be switched to a sixth code,
one of the plurality of base stations includes a timing information sending unit for sending timing information of switching from the fourth code to the sixth code to the first mobile station,
the first mobile station includes a code switching unit for switching the fourth code to the sixth code based on the informing of the code-switching/code-assignment processing unit and the timing information of the one of the base stations,
the one of the plurality of base stations includes a switching unit for switching downlink transmission codes at time when the timing information is sent to the first mobile station, and
the code-switching/code-assignment processing unit includes a code releasing unit for releasing the fourth code used by the first mobile station, at the time when the timing information is sent to the first mobile station.
According to another aspect of the present invention, the code-switching/code-assignment processing unit includes a first code switching unit for informing the first mobile station that a fourth code being used by the first mobile station is to be switched to a sixth code,
one of the plurality of base stations includes a transmitting unit for performing transmission using both the fourth code and the sixth code to the first mobile station,
the first mobile station includes a regenerating unit for receiving both the fourth code and the sixth code and generating reception information by performing maximum ratio combination, a second code switching unit for switching a state of synthesizing maximum ratio to a state of receiving only the sixth code based on own timing, and a switching complete informing unit for informing that code switching has been completed in the first mobile station, and
the code-switching/code-assignment processing unit includes a code releasing unit for releasing the fourth code used by the first mobile station, based on the informing from the first mobile station.
According to another aspect of the code assigning apparatus of the present invention, the code selecting unit is provided in either one of the plurality of base stations or the base station controlling unit.
According to another aspect of the code assigning apparatus of the present invention, the code-switching/code-assignment processing unit is provided in either one of the plurality of base stations or the base station controlling unit.
According to one aspect of a method of assigning codes of the present invention, used in a radio communication system including a base station controlling apparatus, a plurality of base stations, and a plurality of mobile stations, employing CDMA (Code Division Multiple Access) which uses codes of tree structured orthogonal multi-spreading factor sequences as spreading codes, for radio access between the plurality of base stations and the plurality of mobile stations, and providing multi-rate transmission between the plurality of mobile stations and the plurality of base stations, the method of assigning codes comprises the steps of
selecting one or more than one codes corresponding to rate information included in a message from one of the plurality of mobile stations, as first codes,
selecting one or more than one first codes if they are assignable to the one of the plurality of mobile stations, as second codes,
retrieving codes upper than the second codes one by one,
detecting not-assignable codes, which are assigned to another of the plurality of mobile stations, out of the codes upper than the second codes, as third codes,
detecting each level of the third codes,
selecting one of the second codes whose upper third code is in a lowest level in the tree structure, and
assigning the one of the second codes selected by the above step to one of the plurality of mobile stations.
According to another aspect of the method of assigning codes of the present invention comprises the steps of
selecting one or more than one codes corresponding to rate information included in a message from the second mobile station as first codes,
selecting one or more than one first codes if they are assignable to the second mobile station as second codes,
when there is no second code, selecting one or more than one first codes if their lower codes have been assigned to the first mobile station, as fifth codes,
detecting a code lower than the fifth codes and assigned to the first mobile station, as a fourth code,
detecting a code to which switchable from the fourth code for the first mobile station, as a sixth code,
directing the first mobile station to switch from the fourth code to the sixth code, and
assigning a fifth code to the second mobile station after switching all the fourth codes to other codes.
According to another aspect of the method of assigning codes of the present invention comprises the steps of
informing that a fourth code having been used by the first mobile station can be switched to a sixth code,
sending timing information of switching from the fourth code to the sixth code to the first mobile station,
switching the fourth code to the sixth code based on the informing of the informing step and the timing information of the sending timing information step,
switching downlink transmission codes based on the timing information sent to the first mobile station, and
releasing the fourth code used by the first mobile station, based on the timing information.
According to another aspect of the method of assigning codes of the present invention comprises the steps of
informing the first mobile station that a fourth code being used by the first mobile station is to be switched to a sixth code,
performing downlink transmission using both the fourth code and the sixth code to the first mobile station,
receiving both the fourth code and the sixth code,
generating reception information by performing maximum ratio combination,
switching a state of synthesizing maximum ratio to a state of receiving only the sixth code based on own timing,
informing that code switching has been completed in the first mobile station, and
releasing the fourth code used by the first mobile station, based on the informing from the first mobile station.
The above and other objects and features of the invention will appear more fully hereinafter from a consideration of the following description taken in connection with the accompanying drawing wherein one example is illustrated by way of example.